1. Field of the Invention
The present invention relates to a heat-treating apparatus used in the manufacture of a semiconductor device, such as an IC, and adapted for the low-pressure CVD, impurity diffusion and so on.
2. Description of the Prior Art
Many conventional heat-treating apparatuses, e.g., low-pressure CVD apparatuses, are of a horizontal-furnace type in which a furnace body for heat treatment is set horizontally. Recently, however, heat-treating apparatuses of a vertical-furnace type have started to be used, providing the following advantages.
A first advantage of the vertical-furnace type is that the installation area for the apparatus can be reduced.
Naturally, a horizontal-furnace heat-treating apparatus requires an installation area proportional to the length of the furnace. In contrast with this, the installation area for a vertical-furnace apparatus depends only on the diameter of the furnace. In general, the furnace diameter is shorter than the furnace length, so that the installation area for the vertical-furnace type is smaller. Since semiconductor wafers are aligned successively in the furnace during heat treatment, moreover, the furnace length is extended in proportion to the processing capacity, while the furnace diameter has no relation to the capacity. Thus, the higher the processing capability, the greater the advantage of the vertical-furnace type, with respect to the installation area.
A second advantage of the vertical-furnace type is that a vertical convection of gas can be utilized for the supply of working gas to the furnace.
One of the most important requisites of a heat-treating apparatus, whether horizontal or vertical, is the uniformity of temperature distribution along the length of the inside of the furnace. If the temperature distribution is uneven, semiconductor wafers treated in the same lot will be subject to variation in quality. In this regard, the vertical-furnace heat-treating apparatus is inferior to the horizontal-furnace apparatus. This is because heat, in the vertical-furnace type, is transferred upward by the convection, so that the temperature of the top portion of the furnace increases cumulatively. Such accumulation of heat entails thermal fatigue of the heater means.
For avoiding the variation in quality of the wafers treated in the same lot, the heat-treating apparatus is designed so that uniform temperature distribution is attained in a predetermined limited region inside the furnace, and the semiconductor wafers are heat-treated only within this region. This is because a low-temperature region is inevitably created at each end of the furnace, due to an effect of heat radiation to the periphery, so that the internal temperature cannot be made uniform throughout the length of the furnace. In order to improve the production capability of the heat-treating apparatus, it is essential to increase the percentage of a uniform-temperature region. Also in this regard, the vertical-furnace type is handicapped due to the heat convection described above.
One such conventional heat-treating apparatus has means for attaining uniform temperature distribution in a wider region inside the furnace in a steady state. As shown in FIG. 1, this means includes divided heater 2 which surrounds process tube 1. Heater 2, which is formed of an integrated resistance wire, is divided into three portions 3a, 3b and 3c by means of terminals Tl to T4, for example. Currents flowing through resistors 3a, 3b and 3c can be independently controlled by separately setting supply voltages applied between terminals Tl and T2, between T2 and T3, and between T3 and T4. In this prior art apparatus, heating temperatures at both end portions are increased by making the currents flowing through end resistors 3a and 3c greater than the current flowing through central resistor 3b. Thus, the effect of heat radiation at both ends of the process tube is compensated, so that a uniform-temperature region in the central portion is made wider than in the case where all resistors 3a, 3b and 3c have the same heating temperature.
Even with use of the divided heater as described above, however, the efficiency of temperature control at both ends of the process tube is not very high. It is difficult, therefore, to increase the percentage of the uniform-temperature region by shortening resistors 3a and 3c. If resistors 3a and 3c are enhanced in heating temperature and reduced in length, for example, uneven, irregular temperature distribution is observed at the boundary regions between these end resistors and central resistor 3b.The inventors hereof found that this phenomenon is attributable to heat transfer between resistors 3a, 3b and 3c.